1. Technical Field
The present invention relates to an electrochemical energy storage system, and more particularly, to a super capacitor comprising nanostructured metal oxide deposited on metal foam or carbon paper as an electrode and lithium-containing quasi-ionic liquid as electrolyte.
2. Background
Applications for electrochemical energy storage are expanding rapidly as demand grows in various areas including green energy storage and electric-powered transportation. Electrochemical capacitors (ECs) show good potential for high-power applications, but have lower energy density than lithium batteries. As both energy and power densities of a super capacitor relate to the square of the operating voltage, an electrolyte with a large potential domain of stability is crucial. Conventional aqueous electrolyte typically exhibits a potential domain of 1V, limiting its capacitor cell voltage. Non-aqueous electrolytes such as organic solvents do not allow the cell to be operated at high temperatures due to their volatile, flammable, and thermally and electrochemically unstable nature.
The overall performance of a super capacitor depends not only on the selection of electrolytes but also on the selection of electrode materials. For application in a range of energy storage devices, MnO2 has been extensively investigated as a promising electrode material because of its high energy density, low cost, minimal environmental impact, and natural abundance. ECs with MnO2 films as negative electrode and ionic liquid as electrolyte have been investigated and recorded in various prior arts. The research discovered that the cations such as n-butyl-n-methylpyrrolidinium, 1-ethyl-3-methylimidazolium, and 1-butyl-3-methyl-imidazolium adsorb only on the electrode surface and do not penetrate into the [MnO6] octahedral framework. Moreover, in non-aqueous electrolytes, the variation of the oxidation state of Mn in a MnO2 electrode is approximately 0.4, which is smaller than that (˜0.7) observed in aqueous electrolytes.
A small variation in the Mn oxidation state implies that a low percentage of Mn in the structures has undergone reduction-oxidation (redox) reaction, indicating a low rate of ion insertion. This condition is also associated with low electronic and ionic conductivity of MnO2, which kinetically limits the rapid faradaic redox reactions in the bulk materials.
Therefore, the development of a new electrolyte having a large potential domain and a high stability under high temperature is required, and a new electrode with properties that enhance penetration of an electrolyte to compensate the low redox reaction among the charge/discharge cycle is also necessary. Essentially, the desired MnO2 electrode should have better electronic and ionic conductivity. The present invention discloses materials and specific structures to solve the above-mentioned problems in order to improve the capacity performance of conventional ECs.
In the present invention, the maximum energy density of 300 to 450 W h kg−1 obtained from 3D porous metal oxides as an electrode in ionic liquid electrolyte-based EC is notably higher than those of symmetrical (or asymmetrical) ECs based on grapheme ECs (2.8 to 136 W h kg−1), MnO2 nanowire/grapheme composite (MNGC) ECs (30.4 W h kg−1), activated carbon ECs (<10 W h kg−1), or MnO2 nanospheres/carbon nanotubes/polymer composite as an electrode. Moreover, the embodiment of the present invention exhibits a superior power density (˜60 kW kg− at 70 to 120 W h kg−1) and acceptable cycling performance of ˜95% retention after 500 cycles.